1. Field
The present invention relates to the protection of power cables having insulation based on paper impregnated with an oil-based material, and in particular to the joint devices used for joining the ends of power cables in the case in which one power cable has an insulation based on impregnated paper.
The invention may apply advantageously to transition joints or hybrid joints, that is to say for joining two power cables of different technologies, one of the cables having an insulation based on paper impregnated with an oil-based material and the other cable having plastic insulation. The invention may apply to single-conductor cables or to multiconductor cables. It may also apply to joints allowing two power cables having insulation based on impregnated paper to be joined together.
2. Description of Related Art
In joining power cables, generally three principal types of technology are used for producing such joint devices.
In a first technology, called taped technology, the continuity of the electrical functions of the cable is provided by maintaining the continuity of the various layers that make up the cables to be joined. The operation is performed using insulating materials and semiconducting materials in tape form, mainly based on an ethylene-propylene copolymer, preferably an ethylene-propylene-diene terpolymer (EPDM). Another material may then be applied in tape form, based on an ethylene-trifluoroethylene copolymer (ETFE), polyesters or polyamides, such as nylon, or else a silicone-based material. In other applications, an insulation based on a paper tape impregnated with an oil-based material is applied to the EPDM-based tape. Although the use of EPDM-type tape materials is generally satisfactory when the cables have an insulation produced using a viscous impregnation material based on a wax-stabilized oil, for example in MIND (mineral insulated non-draining) cables, it has been found that with cables in which the insulation is made from paper impregnated with relatively fluid oils, these oils pass through the material of the EPDM-type tape, which then undergoes swelling over time and loses its mechanical and electrical properties. The same applies if silicone-based materials are used for such tapes. In all cases, sealing difficulties are encountered at the interfaces of the turns of the helical winding of the tape material, even when the latter is made from materials other than those mentioned above. Finally, the operation of winding these tape materials is tedious and tricky. This represents a substantial practical drawback.
A second technology used for joint devices of this type is what is called heat-shrinkable technology in so far as what is used here is a sheath made of a heat-shrinkable material, for example of the polyvinylidene fluoride (PVDF) type. Heat-shrinking the sheath in order to produce the joint device requires the use of a heating means, which is generally a flame torch, incurring major risks of an accident, especially when one of the cables has an insulation based on paper impregnated with an oil-based material, which may easily catch fire.
A third technology called cold-shrinkable technology consists in using, for the joint, a silicone sheath pre-expanded on a rigid support, the sheath shrinking because of the elastic memory of the material after the rigid support has been removed. However, this technology is not easily applicable in the case of cable having an impregnated-paper-based insulation because of the fact that the silicone sheath is not completely impermeable to the fluid oils that impregnate the paper. In addition, nor is such a silicone sheath sufficiently impermeable to water vapour, thereby reducing the lifetime of the joint and of the paper-based insulation. It is therefore necessary to add additional elements, for example, a rigid tape made of a material impermeable to oil and water vapour, of the ethylene-trifluoroethylene (ETFE) copolymer type with the drawbacks of the above-mentioned taped technology.
It will therefore be understood that the joints for joining power cables in the case in which one of the power cables has an impregnated-paper-based insulation, are difficult to produce and have many drawbacks.